Biodiversity Guidebook Table of Contents]

Natural disturbance type 3:
ecosystems with frequent stand-initiating events

Historically, these forest ecosystems experienced frequent wildfires that ranged in size from small spot fires to conflagrations covering tens of thousands of hectares. Average fire size was likely 300 ha in some parts of the BWBS biogeoclimatic zone, but went as high as 6000 ha in other parts of the zone where topographic features did not limit fire spread. The largest fires in the province occur in this NDT, often exceeding 100 000 ha and sometimes even 200 000 ha.

Natural burns usually contained unburned patches of mature forest that were missed by fire. Consequently, these forests produced a landscape mosaic of even-aged regenerating stands ranging in size from a few to thousands of hectares and usually containing mature forest remnants.

There were also frequent outbreaks of defoliating insects and an extensive presence of root diseases caused by Armillaria and Phellinus (especially in the ICH biogeoclimatic subzones). The impact of these infections on tree survival and stand structure ranged from low to severe. Tree mortality within mature forest remnants and regenerating stands resulted in dead trees, decaying logs, and canopy gaps. Riparian areas within the forest landscape provided special habitat characteristics not found in the upland areas.

Mean return interval for disturbances is about 100 years for the wind-dominated CWH and the fire-dominated SBPS and BWBS with deciduous species prominent. For the SBS and BWBS with coniferous species prominent, the mean fire return interval is about 125 years. The ESSF, ICH and MS units in this NDT experience a mean disturbance return interval of about 150 years.

The presence or absence of Douglas-fir does not influence the disturbance frequency, but determines the number and size of mature remnant stands that survive extensive crown fires to provide structural diversity. Douglas-fir is the most fire-resistant tree species in this NDT.

Biogeoclimatic units in NDT3

The following biogeoclimatic subzones and variants make up this disturbance type:

Seral stage distribution (NDT3)

As a result of the frequent stand-initiating wildfires that occurred in these dry forests, the landscape is characterized by a mosaic of even-aged stands of different ages. Table 10 defines seral stages for each biogeoclimatic zone within this disturbance type; Table 11 recommends targets for seral stage distribution in the type.

Table 10. Seral stage definition for biogeoclimatic zones in NDT3

Table 11. Recommended seral stage distribution for NDT3 (% of forest area within the landscape unit)


Temporal and spatial distribution of the cut and leave areas (NDT3)

Past forest harvesting practices in these areas have produced a landscape pattern that is notably different from the natural pattern. Dispersed medium-sized cutblocks and leave areas have resulted in a fragmented forest with few areas of extensive, contiguous forest. In contrast, beetle-salvage logging has resulted in large-scale disturbances, but without retaining many mature forest remnants within those harvested areas.

A clustered harvest pattern, using large aggregated harvest units, most closely simulates the natural pattern of large fires and large unburned areas or, in certain coastal variants of the CWH, simulates windthrow disturbance. It also results in less fragmentation of the landscape. Retention of patches of forest or single trees within aggregated harvest units simulates the island remnants left within areas of large burns. These remnants are vital to maintain biological diversity, especially when large cuts are used.

A harvest strategy such as this provides numerous ecological benefits. Concentrating harvesting activity in one area allows other large areas of older forest to be left intact and unfragmented for extended periods. As well, the combination of seral stage distribution and harvest unit size recommendations are designed to ensure that some large, unfragmented mature forests are always present on the landscape.

Harvest units and the remaining mature forest stands within the operable forest should be distributed in the landscape unit as shown in Tables 12 to 14.

Table 12. Recommended distribution of patch sizes (harvest units and leave areas)[a] for biogeoclimatic subzones with Douglas-fir throughout stands in NDT3

Table 13. Recommended distribution of patch sizes (harvest units and leave areas)[a] for biogeoclimatic subzones with Douglas-fir restricted or absent in NDT3

Table 14. Recommended distribution of patch sizes (harvest units and leave areas)[a] for alluvial ecosystems in the BWBS biogeoclimatic zone in NDT3


Old seral stage retention and representativeness (NDT3)

The target for old seral stage retention in this natural disturbance type is described in the recommendations below.


Landscape connectivity (NDT3)

In this natural disturbance type, wetland complexes, riparian stands, and the mature forests between them account for most of the connectivity among old seral stage stands. This disturbance type covers a very broad ecological range and has a large degree of variation in the natural connectivity of old and mature forests. The SBPS biogeoclimatic zone in the Cariboo Forest Region and some of the SBS biogeoclimatic subzones in the Prince George Forest Region probably had little connectivity across the forest matrix. Landscape connectivity, however, was provided along riparian corridors. The other biogeoclimatic subzones in this disturbance type (MS, some SBS, ICH, and ESSF) historically had a higher proportion of mature and old forests and a greater degree of old seral stage ecosystem connectivity.

Connectivity can be maintained through the delineation of forest ecosystem networks (see the section “Designing forest ecosystem networks”). It can also be achieved at a broader scale within landscape units, according to the recommendations under “Temporal and spatial distribution of the cut and leave areas,” above. The methods selected should depend on the connectivity objectives of the landscape unit.

Management to reduce fragmentation and maintain connectivity in managed forest landscapes should be guided by the type and degree of connectivity found in each disturbance type. Connectivity can be maintained by a combination of the following methods:


Table 15. The frequency with which connectivity characteristics of natural mature/old seral stage ecosystems occur for all biogeoclimatic subzones of NDT3

If an intermediate or higher biodiversity emphasis is chosen, the areas that are identified as old seral linkages may be incremental to the areas indicated in Table 11. If a lower biodiversity emphasis is chosen, linkages should not result in the areas of old seral stage exceeding objectives in Table 11.

Stand structure (NDT3)

Management for even-aged stands is important for maintaining biodiversity. Therefore, where present, large old Douglas-fir and larch trees should be maintained during forestry operations because they provide structural diversity in this disturbance type. In addition, a component of older seral stages that did not burn historically should be reserved from cutting.


Species composition (NDT3)

Natural forest succession provides for a mosaic of different successional stages in this NDT. Species composition within these successional stages varies from early seral communities to climax communities. Maintaining that variety of species composition within seral stages is an important component of maintaining biodiversity. Where fire has historically been an important part of ecosystem processes, prescribed burning may be used as a management tool to assist regeneration of fire-adapted species.

Rare ecosystems within the landscape unit also contribute significantly to the richness of species composition and to the maintenance of diversity.

The section “Stand management to maintain biodiversity” recommends stand level practices for maintaining species composition.


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